From the point of view of the users of battery operated mobile terminals such as mobile telephones the stand-by time is of great importance. The stand-by time is determined by the consumption of power from the battery, and one way to increase the stand-by time it is to reduce the power consumption of the mobile terminal in its stand-by mode, i.e. when it is on but not in use for communication purposes.
It is known to turn off the power to units and processes in the mobile terminal such as some of the signal processing blocks. In stand-by mode a major task of the mobile terminal is listening to paging indicators and performing measurements and calculations, which are necessary in searching for possible new cells. An example of necessary measurements is measurement of signal quality from neighbouring cells. Another important and power demanding procedure that is performed by the mobile terminal is cell search, i.e. the search for and identification of one or more new cells that each potentially can be used as a new serving cell, if certain conditions are met.
In known Wideband Code Division Multiple Access (WCDMA) systems some of the synchronisation channels such as the Primary Synchronization Channel (P-SCH), the Secondary Synchronization Channel (S-SCH) and the Common Pilot Channel (CPICH) are used to find and to detect new cells. Briefly, the known methods include the following steps:    1. The P-SCH is used to detect a new cell,    2. When a new cell has been detected, the S-SCH is used to find the timing and the scrambling code group of the new cell, and    3. When the timing of the new cell has been found, the CPICH is used for measuring the signal strength of the new cell.
More information about the basic algorithm for making cell search in WCDMA can be found in E. Wang, T. Ottosson: Cell Search in WCDMA. IEEE Journal on selected areas in communications, 18:8:1470-1782, 2000.
FIG. 3a illustrates schematically temporal aspects of a prior art method of cell search in stand-by mode, i.e. when the mobile terminal is turned on but not in use for communication purposes. The figure illustrates the occurrence of remote cell paging indicators, serving cell paging indicators and periods in which the receiver is turned on (‘receiver on’ periods) as a function of time t. When operating in the serving cell with the serving base station 20, whether in stand-by mode or not, the mobile terminal searches for possible new cells among the remote cells to connect to, should it be required. This is called cell search.
The receiver in the mobile terminal must receive the paging indicators 21 transmitted by the serving base station in the serving cell at regular time intervals P, typically every 0.5-1 s. FIG. 3a also illustrates paging indicators 31 from a remote cell. The receiver is turned on or activated periodically in periods 11 all having the same duration τ to receive the signals comprising paging indicators 21 from the serving base station, and reception is started each time a paging indicator 21, typically 300 μs long (half a slot), is to be received. In order to make a good and meaningful reception of the paging (mainly channel estimation) the receiver should be on for typically 3-5 slots, i.e. for around 2.5 ms. The mobile terminal 10 should preferably receive all paging indicators and react accordingly, and the receiver in the mobile terminal is therefore turned on for at least the duration of the paging indicators 21. The receiver is also receptive to possible signals from remote base stations in remote cells in order to identify possible new cells. Therefore the receiver is turned on for longer periods than the duration of the paging indicators 21. These “receiver on” periods 11 are identified in FIG. 3a. Statistically, a new cell will not be found very often. If a new cell with a stronger signal that the serving cell is within the reception range of the mobile terminal, there are requirements defining how fast a new cell must be detected, if the mobile terminal enters the reception range of a new cell with a stronger signal than the serving cell. In order to fulfil the specification requirements (around 5 s in stand-by mode is a typical value) for detecting a new cell, if new cells are present with stronger signals from the corresponding base stations than from the base station in the serving cell, a total signal reception time of around 80 ms is needed. Distributing the 80 ms over paging indicators (1 per 0.5 s) in a 5 s time window gives a signal reception of around a frame, i.e. 10 ms around every paging indicator 21. In this prior art method the receiver is turned on for periods of the same duration, typically 10 ms, whether or not a new cell has been indicated. This is power consuming. FIG. 3a only illustrates the principle but does not reflect the correct relationship between the durations of the involved periods with the receiver on and off, respectively. In practice the duration τ of the periods with the receiver on will be shorter than illustrated.
FIG. 3b shows an enlarged view of an interval of duration P between two punctuated lines in FIG. 3a. In this view, the receiver is turned on (‘receiver on’) during a period 11 with a length of 10 milliseconds. This ‘receiver on’ period is followed by a period with a length of 490 milliseconds in which the receiver is turned off. During the 10 milliseconds period in which the receiver is turned on, a remote cell paging indicator 31 and a serving cell paging indicator 21 is each transmitted during a respective period of 300 microseconds.
FIG. 3c illustrates the timing of two synchronization channels and a common pilot channel during a single frame. One frame is equal to 15 slots or 10 ms or 38400 chips.
The common pilot channel CPICH is transmitted continuously during a slot 34 and during a frame. A first synchronization channel, P-SCH, is transmitted in sequences 32 of 256 chips per slot of 2560 chips, thus giving a duty-cycle of 10%. A second synchronization channel, S-SCH, is transmitted synchronously with the P-SCH channel also in sequences of 33 of 256 chips.
Thus a paging message has a duration corresponding to the duration of half a slot.
It is a purpose of the invention to provide a cell search method that can save battery power in stand-by mode and thus extend the stand-by time of mobile terminals when operating in a wireless cellular communication network.